1. Field of the Invention
The invention generally relates to a portable absorption spectrometer for testing a liquid sample, and more particularly to a near UV absorption spectrometer for determining and monitoring chemicals, especially biocide, in solutions or running water or the like.
2. Description of Related Arts
A biocide is a chemical substance, such as pesticides, which can be fungicides, herbicides, insecticides, miticides, or rodenticides, etc., capable of killing different forms of living organisms used in fields such as agriculture, forestry, and mosquito control. Biocides can also be added to other materials (typically liquids) to protect the material from biological infestation and growth. For example, certain types of quats can be added to pool water or industrial water systems to act as an algicide, protecting the water from infestation and growth of algae. Chlorine can be added in low concentrations to water as one of the final steps in wastewater treatment as a general biocide to kill micro-organisms, algae, etc. Adding hypochlorite solutions to pools, etc. to gradually release hypochlorite and chlorine into the water. Compounds such as sodium dichloro-s-triazinetrione (dihydrate or anhydrous), sometimes referred to as dichlor, and trichloro-s-triazinetrione, sometimes referred to as trichlor, are even more convenient to use. These compounds are stable while solid and may be used in powdered, granular, or tablet form. When added in small amounts to pool water or industrial water systems, the chlorine atoms hydrolyze from the rest of the molecule forming hypochlorous acid (HOCl) which acts as a general biocide killing germs, micro-organisms, algae, etc. Chlorinated hydantoin compounds are also used as biocides.
Restaurants soak and wash cooking ware and silverware in detergents, then rinse away the detergents with water. Thereafter, the ware is soaked in and sanitized with a sanitizing solution. The detergent is a compound, or a mixture of compounds to assist cleaning. Such a substance, especially those made for use with water, may include any of various components having several properties: surfactants to “cut” grease and to wet surfaces, abrasives to scour substances to modify pH, either to affect performance or stability of other ingredients, or as caustics to destroy dirt, water “softeners” to counteract the effect of “hardness” ions on other ingredients, oxidants (oxidizers) for bleaching and destruction of dirt materials other than surfactants to keep dirt in suspension, enzymes to digest proteins, fats, or carbohydrates in dirt or to modify fabric feel ingredients, surfactant or otherwise, modifying the foaming properties of the cleaning surfactants, to either stabilize or counteract foam plus ingredients having other properties to go along with detergency, such as fabric brighteners, sofleners, etc., and colors, perfumes, etc. Quaternary ammonium cations (QAC), also known as quats, are commonly used as sanitizer and have positively charged polyatonaic ions of the structure NR4+ with R being alkyl groups. Unlike the ammonium ion NH4+ itself and primary, secondary, or tertiary ammonium cations, the quaternary ammonium cations are permanently charged, independent of the pH of their solution. Quats in a sanitizing solution are gradually decreased by their combination with the residual detergent. There are legal requirements for the quats concentration in the sanitizing solution to safeguard public health. Inspectors form public health authorities visit restaurants to test with a disposable testing kit or paper so as to ensure the restaurants comply with the concentration standard. If not, the restaurants will be fined. Currently, restaurants dispose of the sanitizing solution either after a certain number of times of use, or after periodic testing shows the quats concentration drops below the standard.
There is a need for a device and method for automatically and economically testing the sanitizing solution for quats concentration.
The prior art applies acid-base titration to measure concentration of quats which makes use of the neutralization reaction that occurs between acids and bases. First of all, a burette should be rinsed with the standard solution, a pipette with the quats solution, and the conical flask with distilled water. Secondly, a known volume of the quats solution is taken with the pipette and placed into the conical flask, along with a small amount of the indicator. The burette should be filled to the top of its scale with the known solution. The known solution is allowed out of the burette, into the conical flask. At this stage, conducting a rough estimate of the amount of this solution it took to neutralize the quats solution. Let the solution out of the burette until the indicator changes color and then record the value on the burette. This is the first titre and should be discluded from any calculations. When all quats have reacted, the solution will have a pH dependant on the relative strengths of the acids and bases. A Quat indicator is in a deprotenated form, and hence carries a negative charge. It thus associates with the quat (a positive ion) to form a complex which changes the pH, the pi electrons' environment and hence the color of the indicator. Then, when all the quats are titrated, the indicators are no longer associated with the quats thus revert to the color they would be in a normal pH˜7 solution (violet/blue and orange, which makes gray).
There are other techniques used to quantify the concentration of QACs. One technique is a procedure developed by Epton which involves a dye-transfer in immiscible solvents, usually chloroform and water. An anionic surfactant such as sodium dodecyl sulfate is used as the titrant and an anionic dye, methylene blue for example, is used to indicate the titration endpoint when the dye transfers color from one phase to the other. The use of chloroform is discouraged because of its toxicity and this technique is not generally used in field applications. References to the original method developed by Epton are: S. Epton, Nature, 160, 795 (1947) S. Epton, Trans, Faraday Soc., 44, 226 (1948).
Another method is the direct titration with sodium tetraphenylborate. QACs suppress the acid color (red) of methyl orange. The addition of sodium tetraphenylborate complexes the QAC and makes the dye color visible. Bromophenol blue exhibits a similar response mechanism turning purple at the endpoint of the titration.
A halide determination is also used to determine the QAC concentration. QACs are cationic molecules with a negatively charged counter ion such as chloride (a member of the halide group in the periodic table). One such halide determination technique for QACs precipitates chloride from acidified QAC solution using silver nitrate. The sample is filtered after the addition of silver nitrate and the filtrate is titrated with ammonium thiocyanate in the presence of ferric ammonium sulfate (Volhard indicator) to the first appearance of pink.
Metrohm AG is a company that specialized in ion analysis describes a method that employs a surfactant ion selective electrode (ISE). The ISE is a liquid membrane electrode optimized for ionic surfactants through careful control of the ionophore/plasticizer that makes-up the electrode membrane. The potential generated by the ISE and reference electrodes is proportional to the concentration of the QAC in the sample, following the Nernst equation; E=E0+k·log(C). In this equation k is a proportionality constant and is ideally 59 mV per decade concentration for monovalent ions at 25° C. Titration of the QAC may use an anionic surfactant such as sodium dodecyl sulfate as the titrant. A plot of titrant volume versus ISE voltage yields an inflection point at the endpoint of the titration.
There is a need to directly measure/monitor the concentration of quats automatically, economically, continuously, and with a high sensitivity.
Absorption spectroscopy uses the range of electromagnetic spectra in which a substance absorbs. In atomic absorption spectroscopy, the sample is atomized and then light of a particular frequency is passed through the vapor. After calibration, the amount of absorption can be related to the concentrations of various metal ions through the Beer-Lambert law. The method can be automated and is widely used to measure concentrations of ions such as sodium and calcium in blood. Other types of spectroscopy may not require sample atomization. For example, ultraviolet/visible (UV/Vis) absorption spectroscopy is most often performed on liquid samples to detect molecular content, and infrared (IR) spectroscopy is most often performed on liquid, semi-liquid (paste or grease), dried, or solid samples to determine molecular information, including structural information. Ultraviolet-Visible Spectroscopy or Ultraviolet-Visible Spectrophotometry (UV/VIS) involves the spectroscopy of photons (spectrophotometry). It uses light in the visible and adjacent near ultraviolet (UV) and near infrared (NIR) ranges. In this region of energy space molecules undergo electronic transitions.
An ultraviolet spectrum is essentially a graph (or plot) of light absorbance vs. wavelength in a range of ultraviolet. Similarly, for a given material of species, such as quats, a standard graph of extinction coefficient ε vs. wavelength is available. Such a standard graph would be effectively “concentration-corrected” and thus independent of concentration.
The measured variable is often the light intensity but could also be the polarization state, for instance. The independent variable is often the wavelength of the light, usually expressed as some fraction of a meter, but it is sometimes expressed as some unit directly proportional to the photon energy, such as wave number or electron volts, which has a reciprocal relationship to wavelength.
Molecular electronic transitions take place when valence electrons in a molecule are excited from one energy level to a higher energy level. The energy change associated with this transition provides information on the structure of a molecule and determines many molecular properties such as color. The relationship between the energy involved in the electronic transition and the frequency of radiation is given by Planek's law. The electronic transitions of molecules in solution can depend strongly on the type of solvent with additional bathochromie shifts or hypsochromic shifts.
The instrument used in UV spectroscopy is called a UV spectrophotometer. To obtain absorption information, a sample is placed in the spectrophotometer and ultraviolet at a certain wavelength (or range of wavelengths) is shined through the sample. The spectrophotometer measures how much of the light is absorbed by the sample. The intensity of light before going into a certain sample is symbolized by I0. The intensity of light remaining after it has gone through the sample is symbolized by I. The fraction of light transmittance is (I/I0), which is usually expressed as a percent Transmittance (% T). From this information, the absorbance of the sample is determined for that wavelength or as a function for a range of wavelengths. Sophisticated UV spectrophotometers can perform automatically. However, such UV spectrophotometers have very complicated structures, very costly, and usually bulky (not portable), for example, DU® Series 500 UV/Vis Spectrophotometer by Beckman Coulter, Inc. (Fullerton, Calif.).
Although the samples could be liquid or gaseous. A transparent cell, often called a cuvette, is used to hold a liquid sample in the spectrophotometer. The pathlength L through the sample is then the width of the cell through which the light passes through. Simple (economic) spectrophotometers may use cuvettes shaped like cylindrical test tubes, but more sophisticated ones use rectangular cuvettes, commonly 1 cm in width. For just visible spectroscopy, ordinary glass cuvettes may be used, but ultraviolet spectroscopy requires special cuvettes made of a UV-transparent material such as quartz.
UV absorption spectroscopy was never applied to directly measure/monitor quats concentration in a sanitizer solution.